1. Field of the Invention
The present invention relates to an illumination device which illuminates an illuminated zone with a coherent light beam, a projection device which projects a coherent light beam, a projection-type image display device which displays an image by using a coherent light beam, and more particularly, an illumination device, a projection device, and a projection-type image display device capable of allowing occurrence of speckles to be inconspicuous.
2. Description of Related Art
A projection-type image display device including a screen and a projection device which projects an image light beam on the screen has been widely used. In a typical projection-type image display device, a two-dimensional image as a original image is generated by using a spatial light modulator such as a liquid crystal micro display or a DMD (digital micromirror device), and the two-dimensional image is magnified and projected on a screen by using a projection optical system, so that an image is displayed on the screen.
As the projection device, various types including a commercialized product called an “optical type projector” have been proposed. In a general optical type projector, the spatial light modulator such as a liquid crystal display is illuminated by using an illumination device including a white light source such as a high pressure mercury lamp, and an obtained modulation image is magnified and projected on the screen by using lenses. For example, JP2004-264512A discloses a technique where a white light beam generated by a supper-high pressure mercury lamp is divided into three primary color components R, G, and B by a dichroic mirror, the light beams are guided by spatial light modulators corresponding to the primary colors, generated modulation images corresponding to the primary colors are combined by a cross dichroic prism to be projected on the screen.
However, a high intensity discharge lamp such as a high pressure mercury lamp has a relatively short lifecycle, and in the case where the lamp is used for an optical type projector or the like, the lamp needs to be frequently replaced. In addition, since a relatively large optical system such as a dichroic mirror is needed in order to extract the light beams of the primary color components, there is a problem in that the size of the whole device becomes large.
In order to cope with this problem, a type using a coherent light source such as a laser is also proposed. For example, a semiconductor laser which is widely used in industries has a very long lifecycle in comparison with the high intensity discharge lamp such as a high pressure mercury lamp. In addition, since the laser source is a light source capable of generating light having a single wavelength, a spectroscopic device such as a dichroic mirror is unnecessary, so that there is an advantage in that the whole device can be miniaturized.
On the other hand, in the type using the coherent light source such as a laser source, there is another problem in that speckles occur. The speckle is a punctate pattern which occurs when the coherent light beam such as a laser beam is irradiated on a scattering surface. If the speckle occurs on the screen, it is observed as punctate luminance unevenness (brightness unevenness), so that it becomes a factor of exerting physiological bad influence on the observer. The reason why the speckles occur in the case of using the coherent light beam is that the coherent light beams reflected from portions of the scattering reflection surface such as a screen have very high coherency, and the speckles are generated through interference therebetween. For example, in a literature “Speckle Phenomena in Optics, Joseph W. Goodman, Roberts & Co. 2006”, theoretical review of the occurrence of speckles is made in detail.
In the type of using the coherent light source, since there is an intrinsic problem in that the speckles occur, techniques for suppressing the occurrence of speckles have been proposed. For example, JP6-208089A discloses a technique where a scattering plate is irradiated with a laser beam, an obtained scattered light beam is guided to a spatial light modulator, and the scattering plate is driven to rotate by a motor, so that speckles are reduced.
With respect to the projection device and the projection-type image display device using the coherent light source, the techniques for reducing speckles which have been proposed up to now as described above, may not effectively and sufficiently suppress the speckles. For example, in the method disclosed in JP6-208089A described above, since the laser beams irradiated on the scattering plate are scattered, a portion of the laser beams are lost without contribution to image display. In addition, although the scattering plate needs to be rotated in order to reduce the speckles, the mechanical rotation mechanism becomes a relatively large device, and the power consumption is increased. Moreover, although the scattering plate is rotated, the position of the optical axis of the illumination light beam is not changed, so that the speckles occurring on the screen due to the diffusion may not be sufficiently suppressed.
In addition, the problem of speckles is not a problem peculiar to only the projection device or the projection-type image display device, but the speckles cause a problem in various devices combined with an illumination device which illuminates an illuminated zone with a coherent light beam.
By the way, stereoscopic display, this is, 3D display by using a projection-type image display device has been commercially performed, for example, in a movie theater or the like. Particularly, in these days, stereoscopic display in TV used at home has drawn attentions. As a method of 3D-displaying an image, a method called an “active shutter glasses type” and a method called a “passive polarized glasses type” have been practically provided in the related art.
In the “active shutter glasses type”, right-eye images and left-eye images are displayed in a time vision manner, and an observer wears dedicated glasses which are opened and closed corresponding to time-divisionally displayed images to observe the images. In other words, during the displaying of the right-eye image, the dedicated glasses open a right-eye window (right-eye lens inserted portion of generally-called glasses) so as to enable right-eye observation and close a left-eye window so as to disable left-eye observation. Similarly, during the displaying of the left-eye image, the dedicated glasses open the left-eye window so as to enable left-eye observation and close the right-eye window so as to disable right-eye observation. In general, the opening and closing of each window of the dedicated glasses are performed by a so-called liquid crystal panel combined to the dedicated glasses. Accordingly, in the “active shutter glasses type”, the dedicated glasses become expensive, so that, in the case where many persons observe the image, a large number of the expensive dedicated glasses need to be prepared. In addition, since the operations of the dedicated glasses require electric power, the dedicated glasses need to be charged in advance, or the image needs to be observed in a state that the dedicated glasses are connected to power supply.
On the other hand, in the “passive polarized glasses type”, the right-eye images and the left-eye images are displayed by using image light beams having different polarization components, and an observer wears dedicated glasses where a polarizing plate which selective transmits right-eye image light beams is attached to a right-eye window and a polarizing plate which selective transmits left-eye image light beams is attached to a left-eye window to observe the images. Therefore, the dedicated glasses for the “passive polarized glasses type” become much more inexpensive than the dedicated glasses for the “active shutter glasses type”, and the electric power is unnecessary. However, in the case where the “passive polarized glasses type” is employed as a projection-type image display device, it is necessary to prevent a polarization state of the image light beam on a projection surface (for example, a screen) from being irregularly disturbed. In other words, in some cases, the “passive polarized glasses type” may not be employed according to properties of the projection surface.
In addition, polarization of a laser beam which is oscillated from a laser source can be allowed to be uniform. Therefore, in the case of employing the “passive polarized glasses type”, although the laser beam may be expected to be used as an image light beam, in the case where the laser beam is used as an image light beam, the above-described problem of speckles occurs.